A novel process for democratised fertiliser production

RESEARCHERS from the University of Tokyo (UTokyo), Japan have developed a bench-scale ammonia production process that they hope could help “democratise” the production of fertilisers.

Ammonia is used as a raw material for nitrogen fertilisers, making ammonia production from nitrogen gas an important industrial process. Ammonia is mainly produced using the Haber-Bosch process which requires high temperature (400–600°C), high pressure (10.1–20.3 MPa), and is very energy intensive. The process consumes 3–5% of all the natural gas that is produced, about 1–2% of the world’s energy supply, according to Yoshiaki Nishibayashi, Professor in the School of Engineering at UTokyo.

Nishibayashi and his team developed the Samarium-Water Ammonia Production (SWAP) process which uses a novel molybdenum-based catalyst to produce ammonia with 90% efficiency. It is small-scale and uses readily-available lab equipment.

SWAP is not currently suitable for industrial scale, but it does demonstrate an opportunity for further research into catalytic nitrogen fixation.

As in the Haber-Bosch process, SWAP uses N₂ from the air. The novel molybdenum-based catalyst combines the N₂ with a proton (H⁺) from water (or alcohols) and an electron from samarium diiodide (SmI₂). SmI₂ can be recycled using an electrochemical method to replenish the lost electrons. In the future, the researchers hope to use renewable sources for the required electricity generation.

The team at UTokyo was strongly motivated to make SWAP “desktop scale” and they achieved a process which can be performed by anyone with the required source material using a “table-top chemistry lab”. Nishibayashi hopes that SWAP will help to democratise fertiliser production.

SWAP produces 4,350 ammonia molecules in about four hours before the enzymes lose catalytic activity. According to Nishibayashi, the rate of production is close to the rate observed with nitrogenase enzymes. In nitrogen-fixing bacteria that produce ammonia at atmospheric temperatures and pressures, nitrogenase enzymes catalyse the reduction of molecular nitrogen to ammonia.

Furthermore, in comparison to the 90% efficiency of SWAP, “average commercial catalysts yield about 10–30% ammonia based on the hydrogen source in closed reactors,” said Nishibayashi. Unreacted H₂ has to be recycled multiple times to approach 100% efficiency.

The SWAP process offers potential energy savings in the process and in the sourcing of raw materials. For example, unlike the Haber-Bosch process SWAP does not require fossil fuels for H₂ production, said Nishibayashi. However, he added that such losses would be countered by the energy required for the synthesis of samarium reagents.

However, Nishibayashi said that further research could also be done to help decrease the amount of samarium reagents used in the process because samarium is a rare earth element. Whilst at lab scale its use as a reducing agent is not an issue, at industrial scales large amounts of the reagent would be required.

Currently the team is working to develop more effective molybdenum catalysts. “We have already achieved ten times higher catalytic activity than that reported in Nature.”

Nissan Chemical Corporation is the research group’s industrial partner.

Nature: http://doi.org/gfzvhq